Preferably, lighting equipment is to light up as soon as it is turned on. An LED lighting apparatus using LED elements should desirably light up instantly, as well. The LED elements are desired to start lighting up at most one second after the activation of the LED lighting apparatus. Accordingly, also in an LED lighting apparatus incorporating a converter, a control IC is required to start control of LED current within about one second after the start of activation.
FIG. 1 is a diagram showing an example of a circuit configuration of an LED driving device described in Japanese Patent Application Publication No. 2010-282757. The conventional LED driving device shown in FIG. 1 includes an AC power supply AC, an EMI filter 1, a capacitor Cin, a rectifying circuit DB, a transformer TR, a MOSFET (switching element) Qin, a control circuit unit 10, a rectifying-smoothing circuit constituted by a diode D1 and a capacitor C1, a capacitor C3, and an error amplification circuit containing an error amplifier OP. The LED driving device and an LED group load device 3 (LED1, . . . LEDn) constitute an LED lighting apparatus. A MOSFET Q1 and the control circuit unit 10 constitute a control IC 5.
The rectifying circuit DB is a known diode bridge circuit, is connected to the AC power supply AC, rectifies AC input power into pulsating current power in one direction, and outputs the rectified power to the transformer TR. The transformer TR has a primary winding W1, a secondary winding W2, and a tertiary winding W3. The primary winding W1 is connected at one end to the rectifying circuit DB and at the other end to a drain of the MOSFET Q1. The rectifying-smoothing circuit constituted by the diode D1 and the capacitor C1 is connected between both ends of the secondary winding W2. An auxiliary power supply which is constituted by a diode D2 and the capacitor C3 and which provides power supply to the control circuit unit 10 after activation is connected between both ends of the tertiary winding W3.
The MOSFET Q1 is grounded at its source via a resistor Rocp, and connected at its gate to the control circuit unit 10.
A serial circuit of the LED group load device 3 and a current sensing resistor Rd is connected to both ends of the capacitor C1, the LED group load device 3 including n pieces of serially-connected LED elements (LED1, . . . LEDn). An inverting input terminal of the error amplifier OP is connected to the current sensing resistor Rd and the LED element LEDn via a gain adjustment resistor Rs. A serial circuit of a phase compensation capacitor Cf and a phase compensation resistor Rf is connected between the inverting input terminal and an output terminal of the error amplifier OP. The phase compensation capacitor Cf and the phase compensation resistor Rf constitute a phase compensation circuit.
A serial circuit of a resistor R1 and a zener diode ZD1 is connected to both ends of the capacitor C1. A serial circuit of a resistor R3, a resistor R4, and a variable resistor Rv is connected between both ends of the zener diode ZD1. A connection point between the resistor R3 and the resistor R4 is connected to a non-inverting input terminal of the error amplifier OP via a resistor R5. A capacitor C2 is connected between the non-inverting input terminal of the error amplifier OP and the ground.
The output terminal of the error amplifier OP is connected to a connection point between the resistor R1 and the zener diode ZD1 via a serial circuit of a resistor R6 and a resistor R2. The resistor R2 is parallel-connected to a photodiode of a photo coupler PC.
One end of a resistor R7, one end of a resistor R8, and one end of a capacitor C6 are connected to an FB terminal of the control IC 5. The other end of the resistor R7 is grounded via a capacitor C5, and the other end of the capacitor C6 is grounded. The other end of the resistor R8 is connected to a collector of a phototransistor of a photo coupler PC, and an emitter of the phototransistor is grounded.
In the LED lighting apparatus thus configured, the control circuit unit 10 turns on and off the MOSFET (switching element) Q1 to thereby transform the rectified voltage from the rectifying circuit DB into a high-frequency voltage and generate a high-frequency voltage at the secondary winding W2 of the transformer TR. This high-frequency voltage is rectified and smoothed by the diode D1 and the capacitor C1, and a DC voltage thus obtained is applied to the LED group load device 3.
A current thereby flows through the LED group load device 3, lighting up the LED elements LED1 to LEDn. In this event, the error amplifier OP amplifies an error voltage between a voltage at the inverting input terminal and a voltage at the non-inverting input terminal. A current according to the magnitude of this amplified error voltage flows through the photodiode of the photo coupler PC, and the amount of light emitted by the photodiode changes according to the current flowing therethrough. Thus, a current according to the amount of light emitted by the photodiode flows through the phototransistor of the photo coupler PC connected to the FB terminal. In other words, a voltage according to a current detected by the current sensing resistor Rd is fed back to the control circuit unit 10, and the control circuit unit 10 controls the duty cycle for turning on and off the MOSFET Q1 so that the voltage according to the current detected by the current sensing resistor Rd can become a predetermined voltage. Further, the brightness level can be controlled by changing the variable resistor Rv.
In the LED lighting apparatus shown in FIG. 1, even when the AC power supply AC is turned off, the error amplifier OP is not powered off instantly, because the amount of electric charge in the capacitor C1 has a large amount of electric charge and therefore requires a certain time to complete discharging. To solve this problem, an LED lighting apparatus shown in FIG. 2 is used.
The LED lighting apparatus shown in FIG. 2 is provided with a diode D3 and a capacitor C7 in addition to the configuration of the LED lighting apparatus shown in FIG. 1. Moreover, one end of the resistor R1 is connected to a cathode of the diode D3 and one end of the capacitor C7, instead of the connection point between the diode D1 and the capacitor C1. An anode of the diode D3 is connected to an anode of the diode D1 and one end of the secondary winding W2, and the other end of the capacitor C7 is grounded. When the AC power supply AC is turned on, a voltage generated at the secondary winding W2 is rectified and smoothed by the diode D3 and the capacitor C7, and a DC voltage thus obtained is applied to the error amplifier OP. When the AC power supply AC is turned off, the error amplifier OP is powered off instantly, because the capacitor C7 has a small amount of electric charge and therefore completes discharging instantly. Thus, the error amplifier OP is always reset at the power-off, and thereby can avoid unstable operation when the AC power supply AC is turned on again.
As described earlier, in the LED lighting apparatus, the control IC is also required to start control of LED current within about one second after the start of activation.
However, the LED lighting apparatus including a single-stage PFC converter with a power factor correction function in Japanese Patent Application Publication No. 2010-282757 has poor power-supply activation performance since the apparatus is provided with the phase compensation circuit which slows down the response speed of a feedback circuit for feeding the secondary-side output of the transformer TR back to the control circuit unit 10. Moreover, in a case where the apparatus is provided with a brightness control function to continuously change the brightness level, the start of lighting is further delayed particularly when the brightness control level is set to the minimum level.
The present invention provides an LED driving device, an LED lighting apparatus, and an error amplification circuit capable of shortening a time required to start LED-lighting.